Procedures for preparing photographic images in silver by diffusion transfer principles are well known in the art. For the formation of positive silver images, a latent image contained in an exposed photosensitive silver halide emulsion is developed and almost concurrently therewith a soluble silver complex is obtained by reaction of a silver halide solvent with unexposed and undeveloped silver halide of said emulsion. The photosensitive silver halide emulsion is developed with a processing composition which may be spread between the photosensitive element comprising the silver halide emulsion and a second element which may comprise a suitable silver precipitating layer. The processing composition effects development of the latent image in the emulsion, and, substantially contemporaneous therewith, forms a soluble silver complex, for example, a thiosulfate or thiocyanate, with undeveloped silver halide. This soluble silver complex is at least in part transported in the direction of the print receiving layer and the silver thereof is precipitated in the silver precipitating element to form a positive image. Procedures of this description are disclosed, for example, in U.S. Pat. No. 2,543,181 issued to Edwin H. Land. See also Edwin H. Land, One Step Photography, Photographic Journal, Section A, pgs. 7-15, January 1950.
Additive color reproduction may be produced by exposing a photosensitive silver halide emulsion through an additive color screen having filter media or screen elements each of an individual additive color such as red or green or blue and by viewing the reversed or positive silver image formed by transfer to a transparent print receiving element through the same or a similar screen which is suitably registered with the positive image carried in the print receiving layer. As examples of suitable film structures for employment in additive color photography, mention may be made of U.S. Pat. Nos. 2,861,885; 2,726,154; 2,944,894; 3,536,488; 3,615,426; 3,615,427; 3,615,428; 3,615,429; and 3,894,871.
U.S. Pat. No. 3,674,482 issued July 4, 1972, is directed to a silver diffusion transfer film unit which comprises a support carrying on one surface, in order, a layer containing a silver precipitating nuclei, an inert non-nuclei-containing protective layer and a layer containing a photosensitive silver halide emulsion. The purpose of the non-nuclei-containing protective layer is to provide a layer over the transferred silver image after the emulsion layer has been removed subsequent to processing, which protective layer will then be the outermost layer. The material for the protective layer is one which is readily permeable to the processing composition and which will not provide sites for the nucleation of the silver forming the transferred image. A particularly preferred material employed as a protective layer comprises chitosan (deacetylated chitin).
U.S. Pat. No. 4,056,392, issued Nov. 1, 1977 is directed to a diffusion transfer film unit which comprises, in order, an additive color screen, a layer comprising silver precipitating nuclei, a layer comprising a water-soluble cupric salt and a compound selected from the group consisting of chitosan and 2-amino-2-deoxyglucose, and a photosensitive silver halide emulsion layer. By employing a water-soluble copper salt in the chitosan protective layer, an increase in D.sub.max is achieved with substantially no adverse effect on D.sub.min compared to a protective layer composed of chitosan alone.
It is well known in the art to employ layers in film units adapted to allow separation of one portion of a film unit from another subsequent to processing. Such layers are referred to as stripping layers or release layers. For example, in U.S. Pat. No. 3,844,789, it is stated (Col. 4, lines 43-51):
"The stripping layer 3 may be prepared with any of the conventional materials promulgated in the art for such purposes and usually comprises a film forming organic colloid, polymeric material, etc., soluble at neutral and/or alkaline pH with typical representatives including, for example, and without necessary limitation gelatin, PVP (polyvinyl pyrrolidone), PVA (polyvinyl alcohol), carboxymethyl cellulose, gum arabic and the like." PA0 "The stripping layers of the elements may be composed of various materials which are softened or dissolved by water. Thus, they may be composed of any water-soluble colloid (viz. which has a marked solubility in water at 20.degree. C.) of natural or synthetic type. Suitable colloids include water-soluble cellulose derivatives, e.g., low substituted cellulose ethers, such as methyl cellulose, ethyl cellulose, and benzyl cellulose; cellulose esters having solubilizing acid salt groups, e.g., sodium, potassium, and ammonium salts of cellulose acid phthalate; cellulose ether acids, e.g., sodium cellulose glycollate, casein gelatin mixtures; water-soluble synthetic resins, e.g., polyvinyl alcohol and esters."
U.S. Pat. No. 2,615,807 is directed to a method of insolubilizing a stripping layer by treating a polyvinyl acetal with an aqueous solution containing a water-soluble cationic surface active wetting agent and then resolubilizing the layer for stripping by treating with an aqueous solution containing an anionic surface active wetting agent.
Stripping layers are disclosed in U.S. Pat. No. 2,492,952 as follows: (Col. 3, lines 6-20):
The above-mentioned U.S. Pat. No. 3,674,482 also discloses the use of stripping layers and mentions as suitable materials for said stripping layers cellulose acetate hydrogen phthalate or acid functional waxes such as methyl acrylate/acrylic acid copolymer.